I don't know about courses (especially free ones), but here are some good tutorials on making webcomics:
These are for a piece of software called Clip Studio that a lot of comic artists in various styles use. It's usually on sale a few times a year and I recommend getting it if you're serious about making webcomics.
I'd also very strongly recommend picking up a book called "Making Comics" by Scott McCloud:
For reference for figures, a lot of people use a site called Posemaniacs, but it's also worth considering that if you do get clip studio, it comes with poseable 3D models inside the app. I would say though that it's good to use 3D models alongside practicing drawing anatomy from life and without 3D models too. Use them as help and reference, but don't become too reliant on them if possible.
For watercolour tutorials, I'd actually recommend google searching "[name of the app you're using] watercolour tutorial" because you might find the best guidance and even links to brushes and textures to help you.